Borehole measurement devices may be used to determine formation characteristics surrounding the borehole and are typically used in wellbores drilled for the purpose of extracting natural resources, such as hydrocarbon, from the formations surrounding the borehole. Borehole measurement devices may use different types of measurements, for example, a borehole measurement device may use gamma measurements, thermal neutron measurements, resistivity measurements or other types of measurements.
At present there are a number of physics being employed to perform thru pipe formation evaluation and other wellbore measurements. For neutron based measurements, there are pulsed neutron thermal neutron, pulsed neutron gamma, gamma gamma, neutron thermal neutron, neutron epi-thermal neutron etc. It is noted that naming convention for down hole geophysical devices are based on source—detection physics. For example neutron thermal neutron indicates a neutron source and thermal neutron detection. Most of these systems utilize one source per physics, this will be referred to as single physics measurement. Systems that utilize one source for two physics will be referred to as a dual physics measurement. Of the currently available dual physics systems, there is one that utilizes a combination neutron thermal neutron, neutron epi-thermal neutron. In addition there is a neutron thermal neutron, neutron gamma dual physics measurement device built in Azerbaijan.
Some shortcomings or disadvantages of using single physics measurements is the lack of corrections available for factors such as borehole rugosity, annular fluid changes, mineralogy, tubulars etc. To compensate for these short comings, dual detectors are used. These devices are commonly referred to as compensated devices. Another solution is to combine multiple single physics measurement devices, including compensated devices, during analysis. An example is neutron thermal neutron and gamma gamma physics, commonly referred to as neutron density measurements. Measuring thru pipe also limits the effectiveness of some of the physics. For example, gamma gamma measurements are limited because the pipe itself shields gamma and therefore there are losses as the gamma photons travel from the source thru the pipe and then again as the photon returns back to the detector. This results in low count rates and increases error in the measurements.
Neutrons can easily penetrate pipe and therefore is a logical choice for thru pipe measurements. Of the neutron physics based measurements; pulsed neutron devices do not lend themselves to dual physics measurements. The reason is that the length of the pulsed neutron source does not allow for effective measurement spacing for the required detectors. Chemical neutron sources are much smaller and therefore can be used effectively for dual physics based measurements. The neutron epi-thermal measurement is highly sensitive to borehole rugosity and therefore not an ideal choice to determine formation parameters.
A measuring tool or logging device together with a method of carrying out formation evaluation using same is therefore needed.